1. Field of the Invention
This invention relates to a four-wheel steering system for a vehicle in which both the front wheels and the rear wheels are turned in response to operation of the steering wheel.
2. Description of the Prior Art
In the four-wheel steering system which is to be put into practice in the near future, it is preferred that the turning angle of the rear wheels for a given turning angle of the steering wheel be changed in steps. Thus, there is required a steering ratio changing mechanism which is disposed between the steering mechanism and the rear-wheel turning mechanism to change the ratio of the turning angle of the rear wheels to the turning angle of the steering wheel. In U.S. Pat. No. 4,313,514, there is disclosed a steering ratio changing mechanism in which a lever is utilized for changing the ratio of the turning angle of the rear wheels to the turning angle of the steering wheel. (This ratio will be referred to as the "rear wheel steering ratio", hereinbelow.) In the steering ratio changing mechanism, a pivoted lever is connected to the steering mechanism at one end and to the rear wheel turning mechanism at the other end, and the rear wheel steering ratio is changed by changing the position of the pivotal center of the pivoted lever.
However this system is disadvantageous in that since the pivoted lever must be supported both for pivotal movement and sliding movement, backlash is apt to be produced in the supporting portion, and in that it is difficult to obtain sufficient steering rigidity since the supporting portion is subjected to a counterforce the magnitude of which depends upon the lever ratio. Further, the input force imparted, in response to operation of the steering wheel, to the pivoted lever which is large in length acts as a large bending force and accordingly it becomes difficult to obtain sufficient steering rigidity.
Further, in the steering ratio changing mechanism of the patent publication, as the angle formed between the axial direction of the pivoted lever and the direction of input from the steering mechanism is reduced, the pivoted lever tends to be displaced with respect to the supporting portion, which is not preferred in view of the desire to obtain a precise steering ratio.